1. Field of the Invention
This invention relates to an improved control system for energizing a stepping motor; and, more particularly, to a system which is efficient in terms of the overall power consumed by the motor.
2. Description of the Prior Art
Stepping motors of the type to which this invention is applicable are well known in the art. Such stepping motors have a number of angular rotor positions at which the rotor is balanced. An electrical energy pulse causes the rotor of the motor to step from one balanced position to the next. The direction of rotation of the motor is determined by the sequence in which the field coil windings are energized as the total angle through which the rotor travels is a function of the number of pulses supplied to the field coil windings.
As will be appreciated by those skilled in the art, such stepping motors are advantageous in that the angular position of their rotor can be simply and accurately determined by counting the number of pulses supplied to the field coil windings. Such motors are used widely in numerical control machine tool systems and also in other process control systems. In the case of a process control system, for instance, the position of the stepping motor rotor can be varied as a function of the difference a feedback signal from a rebalancing element between a process variable signal and in order to rebalance the system.
In order to operate satisfactorily in most control systems, a stepping motor must usually develop a large torque as it initially steps from one balance state to a new balance state. the inertia or friction of the motor's rotor and the inertia or friction of the connected load call for this large torque. In addition, in control system applications, it is often desirable that the delivered torque vary as the motor steps through a series of positions.
It will further be appreciated that in a control system application the motor may sit at its balance point for considerable periods of time. If the field coils are energized during these periods, the system will be inefficient in terms of power consumption and also the motor will tend to overheat.